Low temperature poly-silicon (LTPS) technique is a new process for manufacturing a thin film transistor (TFT) display device, and an LTPS TFT display device may provide an image with well display quality due to its rapid response and high resolution. When the LTPS technique is adopted for forming a peripheral circuit of the display device, the number of integrated circuits (ICs) can be reduced and a structure at a periphery of the display device can be simplified, thereby providing a narrow-bezel product.
Although with great development in the LTPS technique, there still exist some defects for the LTPS TFT, i.e., it is impossible to effectively suppress the occurrence of a leakage current, and excessive heat will be generated. The excessive heat is generated by the LTPS TFT due to impact ionization caused by electrons accelerated under the effect of a relatively large horizontal electric field of the LTPS TFT. Because of the excessive heat, lattice scattering will occur, so metal ions, e.g., Na+, in a glass substrate will be diffused toward an active region and thereby a threshold voltage (Vth) of the LTPS TFT will be adversely affected. In addition, a negative resistance effect will occur at a saturation region, so the carrier mobility and the on-state current will be reduced. Furthermore, when the excessive heat is generated for a long period of time, the characteristics of the LTPS TFT will be deteriorated and the product quality will be adversely affected.
It has been found through research that it is possible to effectively reduce the generated heat as well as the leakage current by forming the active region of the LTPS TFT to incorporate a lightly-doped drain (LDD). Since the LDD has a relatively high resistance, which is equivalent to a serially-connected resistor of high resistance, the intensity of the horizontal electric field of the LTPS TFT can be reduced, and the distribution of the electric field in a channel of the LTPS TFT can be improved, thereby reducing the generated hot carriers due to impact ionization caused by electric field acceleration and effectively suppressing the occurrence of the leakage current.
However, there is such a dilemma in the related art that, the LDD will lose its capability to reduce the generated heat and suppress the leakage current when the LDD is too short, but the power consumption of the LTPS TFT will be increased and an aperture ratio of the display device will be adversely affected when the LDD is too long.